The present invention, generally, relates to the field of packaging for electronic circuits and, more particularly, to a new and improved composite dielectric structure for electronic packaging of circuits and devices.
For conventional modules that utilize a thin film dielectric to separate two metal levels, the flexibility for variation in electrical parameters is extremely limited. For example, one thin film process has a polyimide dielectric thickness equal to 0.36 mils. This structure is characterized by low coupling noise, high drive capacitance and a low impedance.
A typical thin film process is photolithographic in essence and enables the processing of relatively small and refined geometries. In contrast, the typical thick film process is characterized by the use of masks, i.e., moly masks, through which a dielectric material is screened. The thick film process is quite coarse and has a larger geometry relative to the thin film process.
A minimum practical thickness for a layer of dielectric material for the thick film process is 5-mils, with a 1-mil minimum thickness for the metal conductors.
Typically, for the thin film process, the drive capacitance is in excess of 6 Pico-farads per inch for 3 mil wide signal lines and has an impedance of less that 23 Ohms when a solid ground plane is used. The drive capacitance is too high for most circuit technologies, and an impedance as low as 20 Ohms causes reflection problems for the typical 80 Ohms card-on-board applications.